This invention relates generally to the treatment of waste water and deals more particularly with improvements in the application of ultra fine air bubbles to municipal and industrial waste water.
Various types of aeration and mixing devices have been used to treat municipal and industrial waste water, including mechanical devices which function as surface aerators. Surface aerators are active only at the surface of the waste water and do not effectively treat remote areas of the lagoon or basin. Due to the localized nature of surface aeration and inability to mix to the bottom in deep tanks or basins, there is a lack of uniform distribution of energy throughout the waste water and a corresponding lack of thorough mixing of the air with the liquid. Surface aerators are also generally low in efficiency and high in energy consumption, and they are characterized by frequent mechanical problems. In addition, surface aeration systems are lacking in flexibility and are difficult and expensive to maintain in good operating condition.
Aeration systems known as draft tube systems utilize a series of aeration tubes which extend vertically in the waste water basin. The tubes act as draft tubes, and air is applied internally to create air lift pumpage and aeration of pumped liquid. The draft tube systems dispense the air fairly widely throughout the basin but are plagued by high energy requirements. Furthermore, the draft tube system does not stand up well when subjected to prolonged use in the field.
Fine bubble technology is used in a third type of aeration system. In the fine bubble system, small air bubbles are generated and applied to the waste water. The fine bubbles have been generated by mechanical devices such as eductors which draw gas into the liquid by liquid pumpage. Rapidly rotating turbines and pumps have also been used to break up a stream of compressed air into fine air bubbles at the air release point of the system.
Another type of fine bubble aeration device that has been used to great advantage is an aeration unit having a 1 to 11/2 inch thick porous diffuser plate of ceramic or sintered metal through which compressed air is forced. The aerator is submerged at or near the bottom of the waste water treatment basin, and the compressed air that passes through the small pores of the diffuser plate is released in the form of fine air bubbles which rise through the waste water to provide aeration. The diffuser plate aerator is generally more efficient in oxygen transfer than the other types of aerators that have been used, and it disperses the air bubbles more thoroughly throughout the waste water. The effective aeration that is achieved by the fine bubble system is due primarily to the relatively large areas of surface contact between the small air bubbles and the liquid.
Conventional fine bubble aeration systems are designed to maximize residence time of the compressed air bubbles in the liquid in order to maximize the oxygen transfer. The surface area of the diffuser media is normally maximized and liquid pumpage is minimized. The goal of the system is to widely dispense the air bubbles across the basin, cause the bubbles to rise slowly and independently through the waste water, and minimize directional flow by minimizing the intensity of the air flow. For example, conventional systems operate at an air flow rate of 0-8 cfm per square foot of diffuser media. Most common rates are 1 to 3 CFM per square foot.
Even though the approach taken by conventional systems maximizes the oxygen transfer of the air that is pumped through the diffuser media, there are no significant surface aeration effects since the slow rise rate of the bubble does not creatae appreciable turbulence at the surface. The non-directional effect resulting from the wide dispension and low rate of air flow lead to mixing problems and inefficiencies, particularly in lo F/M systems where the F/M ratio is less than about 0.2 #BOD/#MLSS. In high rate activated sludge systems having a higher F/M ratio, existing fine bubble systems mix in an efficient and satisfactory manner for the most part. However, in low rate activated sludge systems or in aerated lagoon systems where the F/M ratio is extremely low, the oxygen demand can be satisfied at low energy levels which do not result in enough mixing to achieve proper interaction between the food and microorganisms. Therefor, in order to provide sufficient mixing to sustain the process, energy must be added beyond that required for aeration, and the energy requirements are increased accordingly. Net energy requirements for the conventional fine bubble system are substantially greater than the theoretical energy to dissolve oxygen.
All of the fine bubble aeration systems that have been available in the past for lagoons and low F to M ratios require rigid pipes and are thus rather costly to construct and install. Plastic pipe has not been used, primarily because of its susceptibility to significant expansion and contraction due to temperature changes. Attempts to use plastic pipe in lagoons or low F/M basins which fluctuate widely in temperature have been unsuccessful and have resulted in pipe breakage and failed connections caused by thermally induced expansion and contraction of the pipes when the temperature changes. Plastic pipe can expand and contract enough (several feet in some cases) to make it impractical for use in conventional fine bubble aeration systems which employ rigid connections throughout and are not able to accommodate thermal expansion and contraction of the pipe in lagoons and low F/M basins which fluctuate in temperature.
Conventional fine bubble systems for activated sludge applications (high F/M ratios) are also characterized by high initial costs and high operating costs. The aerators are arranged closely together and a large number of aerators is required to adequately treat a large basis. Lenghty rigid pipe air lines are necessary to hold the aerators, along with a large number of connecting devices for joining the aerators (diffusers) to the air lines. The diffuser units can become clogged so severely from build up of deposits that application of chemicals such as hydrochloric acid is required to clear them. The aerator devices which are submerged at the bottom of the basin are inaccessible and difficult to service. The porous diffuser media are often ceramic or sintered metal plates which have rough texture surfaces that offer little resistance to biological growth tending to clog their pores. The diffuser plates are normally thich enough (1 to 11/2 inches) that the pores present tortuous paths which are easily clogged by debris in the air supply, again blocking the diffuser and reducing the effectiveness of the aeration.
Fine bubble systems have not been successfully applied to aerated lagoons. To employ conventional fine bubble system requires rigid air piping, strict elevation control of lagoon bottom, elevation control of air line, and strict elevation control for diffuser units. Costs to provide these features have been prohibitive because of the very large areas in lagoons. Air flow rates for fine bubble air systems are so low per unit that massive piping systems would be required.
Fine bubble aeration systems traditionally require considerable maintenance. Lagoons are not usually constructed in parallel to allow units to be removed from service for maintenance. Also, lagoons are so large that dewatering for maintenance is not practical. Since fine bubble systems are fixed in place and cannot be serviced or removed from the lagoon without dewatering, fine bubble aeration has not been a viable treatment option even though substantial energy savings would be possible. The fear of operating and maintaining fixed in place fine bubble systems has effectively prevented their application to lagoons.
The aerators that have been used in fine bubble systems have also been unsatisfactory in a number of respects. Aerator units which employ stone (ceramic) media typically have the diffuser permanently bonded in place so that it cannot easily be replaced. Holding the aerator down and in place without air leakage has also been a problem.